There are certain HP-1s with a slightly different driver. These were sold in black, rather than silver, boxes and have anisotropic magnets, which appear to be the normal ferrite discs used in most HP-1s but with thin, perforated iron or steel plates attached to the outer surfaces of the driver, which very much resembles the later YH-100 driver but is a couple of dB less efficient. There was no model change on the headphones themselves, just the appropriate change in the magnet specification, and the sensitivity (efficiency) showed a 2dB gain (see below-- also note that the service manual gives only the sensitivity of the Anisotropic model). It is uncertain so far what, if any, sound differences these drivers exhibit."